You are best to check with your physician before trying anything new, especially with asthma.
C~
I would first use transdermal Magnesium Oil before nebulized MgCl Oil.
Apply to your entire body -excluding sensitive areas-massage well and leave on for 20-30' before jumping into the shower. Repeat every other day.
It is the safest and most efficient way to build up your Mg stores.
If you like to know how to make your own Magnesium Oil, let me know.
Have you looked into the Buteyko Breathing Technique for Asthma?
If not, just search it.
Take care,
Niko